Polyureas, polyurethanes, and polyurethaneureas are well known to the polymer arts and are made by reacting a polyisocyanate with one or more active hydrogen containing materials. In the case of polyureas a polyamine is reacted with a polyisocyanate to form a polymer containing urea (--NH--CO--NH--) linkages. In the case of polyurethanes a polyhydric alcohol is reacted with a polyisocyanate to form a polymer containing urethane (--O--CO--NH--) linkages. Polyurethaneureas, as the name implies, are made by reacting a mixture of polyamine and polyhydric alcohol with a polyisocyanate to yield a polymer having mixed (urethane and urea) linkages.
Generally, polyamines react with polyisocyanates (to form polyureas) at a much faster rate than polyhydric alcohols (to form polyurethanes) and are thus prime candidates for use in fast reaction injection molding (RIM) systems. In such systems secondary amine groups react more slowly than primary amine groups.
In slower molding reaction systems such as those designed for use with open casting low pressure apparatus, however, even the more slowly reacting secondary amines are generally still too reactive for use therein, and the (polyurea) amine/isocyanate composition tends to cure and "set up" before the mold has entirely filled.
Thus open casting systems tend to be used most often with polyurethane systems. Such systems, comprising a polyol and polyisocyanate as described above, react slowly enough to allow more than adequate mold filling time, but herein also lies a disadvantage in such systems since their in-mold residence (cure) time, i.e. the amount of time before a molded article can be demolded without damage, can be long enough to adversely affect production.
A modified polyurethane casting is disclosed in U.K. Patent Application No. 2160881 published Jan. 2, 1986, the polyurethane material being prepared by the reaction of
(a) an essentially primary hydroxyl tipped polyol having a molecular weight in the range of from 2,500 to 8,000: PA0 (b) a polyhydric alcohol; PA0 (c) an aliphatic monohydric primary alcohol containing up to 10 carbon atoms; and PA0 (d) a polyisocyanate, the reaction being carried out at a isocyanate index of 1.0 or greater. PA0 (A) an aromatic polyisocyanate; and PA0 (B) an active hydrogen-containing composition comprising PA0 (a) polyether polyols and/or hydrocarbon-based polyols having a molecular weight from 60 to 400, and an average hydroxyl functionality from 1.9 to 2.5; PA0 (b) polyether (and/or thioether) polyols having a molecular weight of at least 1000 and an average hydroxyl functionality from 1.9 to 4; PA0 (c) polyester polyols having a molecular weight from 100 to 1000, and an average hydroxyl functionality from 1.9 to 2.5; PA0 (d) polyester polyols having a molecular weight greater than 1000, and an average amine hydrogen functionality from 1.9 to 4; PA0 (e) amine terminated polyethers having a molecular weight of at least 1000, and an average amine hydrogen functionality from 1.9 to 6.
The above U.K. patent application states that the polyurethane materials disclosed therein have certain desirable properties, including being readily deformable, possessing delayed recovery characteristics, and being characterized by low compression set with high, temperature-insensitive damping over a broad temperature range. The present inventors have determined that such systems are, however, disadvantageous in that they require relatively long mold residence times.